Retina

The retina is a thin layer of cells at the back of the eyeball of vertebrates and some cephalopods; it is the part of the eye which converts light into nervous signals.

Physical structure of human retina

In adult humans the entire retina is 72% of a sphere about 22 mm in diameter. At the centre of the retina is the optic nerve. This spot is known as the blind spot as it lacks photoreceptors. It appears as an oval white area of 3 mm2. Temporal (in the direction of the temples) to this disc is the macula. At its center is the fovea, a pit that is most sensitive to light and is responsible for our sharp central vision. Human and non-human primates possess one fovea as opposed to certain bird species such as the hawk who actually are bifoviate and dogs and cats which possess no fovea but a central band known as the visual streak. Around the fovea extends the central retina for about 6mm and then the peripheral retina. The edge of the retina is defined by the ora serrata. The length from one ora to the other (or macula), the most sensitive area along the horizontal meridian is about 3.2 mm.

Related Topics:
Blind spot - Macula - Fovea - Ora serrata - Meridian

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In section the retina is no more than 0.5 mm thick. It has three layers of nerve cells and two of synapses. The optic nerve carries the ganglion cell axons to the brain and the blood vessels that open into the retina. Perhaps as a product of evolution, the ganglion cells lie innermost in the retina while the photoreceptive cells lie outermost. Because of this light must first pass through the thickness of the retina before reaching the rods and cones. However it does not pass through the epithelium or the choroid (both of which are opaque).

Related Topics:
Nerve - Synapse - Ganglion cell - Axon - Evolution - Epithelium - Choroid

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The white blood cells in the capillaries in front of the photoreceptors can be perceived as tiny bright moving dots when looking into blue light. This is known as the blue field entoptic phenomenon (or Scheerer's phenomenon).

Related Topics:
White blood cell - Capillaries - Blue field entoptic phenomenon

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Between the ganglion cell layer and the rods and cones there are two layers of neuropils where synaptic contacts are made. The neuropil layers are the outer plexiform layer and the inner plexiform layer. In the outer the rod and cones connect to the vertically running bipolar cells and the horizontally oriented horizontal cells connect to ganglion cells.

Related Topics:
Neuropil - Plexiform

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The central retina is cone-dominated and the peripheral retina is rod-dominated. In total there are about six million cones and a hundred and twenty-five million rods. At the centre of the macula is the foveal pit where the cones are smallest and in a hexagonal mosaic, the most efficient and highest density. Below the pit the other retina layers are displaced, before building up along the foveal slope until the rim of the fovea or parafovea which is the thickest portion of the retina. The macula has a yellow pigmentation from screening pigments and is known as the macula lutea.

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